A variety of electronic devices, such as computers, monitors, flat panel displays, wireless communication devices, cellular phones, high speed two-way digital transceivers, and paging devices, to name just a few, utilize synchronous signals, such as, clock signals, vertical-synch and horizontal-synch signals, spread spectrum and digital wireless communication signals, etc., that are phase synchronized with other signals associated with such devices. Another variety of electromechanical devices such as compact disc players and digital video disc drives require close tracking of one or more physical elements in a control system. A phase-locked loop circuit is used to perform the described signal synchronization and tracking functions. A phase-locked loop is a closed loop frequency control system. A phase-locked loop generally includes a phase detector, an amplifier, and a controlled oscillator.
Phase-locked loop circuits have been conventionally manufactured using analog circuit construction. An analog phase detector in a phase-locked loop produces an analog output signal, for example a voltage signal, to indicate a phase difference between two signals. In a phase-locked loop, for example, this analog signal may control a frequency source, such as a voltage controlled oscillator (VCO). High precision adjustments in the frequency signal output from the VCO may depend on very precise components and custom analog circuit design when the phase detector is an analog phase detector.
Analog phase-locked loops, as with most analog circuit designs, suffer from sensitivity to noise signals, temperature variability, and manufacturing process variations. Further, increasing the precision of an analog circuit may require significant additional component cost and multiple fabrication iterations. Moreover, analog circuit designs tend to require relatively large circuit footprints to implement a precision phase-locked loop circuit. Additionally, as with any analog circuit design, the design time tends to be long to transfer a design to a new circuit manufacturing process. To transfer an analog phase-locked loop to a new manufacturing process, the design effort and risk are substantially the same as with the original design.
Conventional phase detectors include two basic types. A Type-I phase detector is designed to be driven by analog signals or digital square-wave signals, whereas a Type-II phase detector is driven by digital transitions. In its simplest form the Type-I phase detector (digital) can be implemented using an exclusive-OR gate, the output of which is “on” when a signal voltage differs from a reference voltage. The Type-II phase detector is sensitive only to the relative timing of edges between the reference signal and a second signal. The Type-II phase detector generates either lead or lag output pulses, depending on whether the output transitions from the controlled oscillator occur before or after the transitions from the reference signal, respectively. The width of the lead or lag pulses is equal to the time between the respective edges. Output circuitry either sinks or sources current during those pulses and is otherwise open-circuited. The occurrence of output pulses (or the lack thereof) generated by the Type-II phase detector is independent of the duty cycle of the input signals, unlike what occurs with a Type-I phase detector. Another useful feature of the Type-II phase detector is that the output pulses disappear when the two signals are “locked” or in phase with each other. Consequently, there is no undesired voltage ripple present at the output to add periodic phase modulation in the loop as with the Type-I phase detector. Because the output of a Type-I phase detector is always generating an output wave, the output wave must be low-pass filtered to smooth the output signal. Consequently, ripple and periodic phase variations are present in a loop with a Type-I phase detector.
In circuits where phase-locked loops are used for frequency synthesis, the Type-I phase detector adds phase-modulation sidebands to the output signal. Unlike the Type-I phase detector, which is always generating an output signal, the Type-II phase detector generates output pulses only when there is a phase error between an input signal and a reference signal. Since the phase detector output otherwise looks like an open circuit, a loop filter capacitor acts as a voltage storage device, holding the voltage that generates the correct oscillator frequency. If the frequency of the reference signal changes, the phase detector generates a train of short pulses that charge or discharge the loop filter capacitor to the voltage desired to return the oscillator frequency to the lock frequency.
U.S. Pat. No. 6,429,693 describes a digital fractional phase detector using a delay chain to measure fractional delay differences between the significant edge of a VCO output clock and a reference clock by using a time-to-digital converter to express the time difference as a digital word for use by the frequency synthesizer. The circuit area required to implement the digital fractional phase detector is dominated by the area of the time-to-digital converter, which comprises a plurality of inverters coupled in series with a respective latch or register coupled at the output of each of the inverters. A reference signal is used to clock a respective input into each latch. The output of every other latch is inverted before being forwarded to an edge detector. The circuit area required to implement the digital fractional phase detector is proportional to the maximum detectable phase difference as determined by the number of bits in the converter and inversely proportional to the length of time that can be resolved in the delay portion. Thus, with increasing demands on precision comes a need for additional circuit area.
With the increasing popularity of digital circuits in all of the aforementioned electronic and electromechanical devices, current trends are toward smaller and more compact devices requiring smaller circuit designs, continuous improvements in circuit manufacturing technologies requiring easily adaptable circuit designs for new technologies, and increasing demand for higher precision phase synchronization.
Therefore, it would be desirable to provide a reliable, high-precision phase detector that can be realized using less circuit area than conventional designs.